[bitcoin-dev] Overhauled BIP151

[Jonas Schnelli <dev@jonasschnelli.ch>]

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Hi

During work on the implementation of BIP151 [1] I figured out that the current
published proposal could be further optimized.

I wrote an overhauled BIP151 specification with some – partially radical –
changes.

Now it’s unclear to me if this should be published under a new BIP nr. or if it
is acceptable to change the existing 151 proposal.
If a new BIP number would be required, I think withdrawing BIP151 should be
done (which somehow indicates we should alter 151).

The only BIP151 implementation I’m aware of is the one from Armory [2].
BCoins implementation has been removed [3].

The new proposal draft is available here:
https://gist.github.com/jonasschnelli/c530ea8421b8d0e80c51486325587c52

Major changes
=============
- the encryption handshake no longer requires the v1 protocol, it’s a pure
  32bytes-per-side „pseudorandom" key exchange that happens before anything else.
- the multi message envelope has been removed.
- a new NODE_ENCRYPTED service bit
- the key derivation and what communication direction uses what key is now more
  specific
- the length of a packet uses now a 3-byte integer with 23 available bits
- introduction of short-command-ID (ex.: uint8_t 13 == INV, etc.) which result in
  some v2 messages require less bandwidth then v1
- rekeying doesn’t require a message and can be signaled in the most
  significant bit in the packet-size field


Points that are in discussion and may be added to the BIP (or to a new one):

Hybrid NewHope key exchange
===========================
The current ECDH key exchange is vulnerable to Shor’s algorithm and is thus not
considered quantum-safe.
Following TORs approach [4] by adding a NewHope [5] key-exchange the handshake
protocol would very likely make the encryption PQ safe with little costs.
There is also a straight forward implementation [6] from the NewHope team that
has been submitted to NIST PQC project.

Inefficiency of ChaCha20Poly1305@openssh
========================================
The proposed AEAD could eventually be further optimized.
ChaCha20Poly1305@openssh uses at least three rounds of ChaCha20 which
eventually can be reduced to two (messages below <=64 bytes [inv, ping,
pong,...] only require one round of ChaCha20, but two for the Poly1305 key and
the message length encryption where the Poly1305 key chacha round „throws away“
32 bytes).


I would suggest that we don’t rehash discussions about the general
concept of encrypting the traffic. This has already been discussed [7][8].

I hope we can limit this thread to discuss further ideas for optimisation as well as
technical details of the published proposal or its implementation.


[1] https://github.com/bitcoin/bitcoin/pull/14032
[2] https://github.com/goatpig/BitcoinArmory/pull/510
[3] https://github.com/bcoin-org/bcoin/commit/41af7acfd68b0492a6442865afd439300708e662
[4] https://gitweb.torproject.org/user/isis/torspec.git/plain/proposals/XXX-newhope-hybrid-handshake.txt?h=draft/newhope
[5] https://eprint.iacr.org/2015/1092
[6] https://github.com/newhopecrypto/newhope

[7] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-February/013565.html
[8] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-June/012826.html


Thanks

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